Bottled error distorts N2O estimates.

Bottled error distorts N.sub.2.O estimates

In the dentist's office it goes by the name of "laughing gas," yet nitrous oxide (N.sub.2.O) is no laughing matter in the atmosphere, where it serves as a "greenhouse" gas and leads to the destruction of stratospheric ozone. As levels of this gas rise by some 0.2 to 0.3 percent annually, scientists are trying to determine how much each major source of it contributes to the atmospheric burden. Recent work has suggested that power plants -- particularly those that burn coal -- contribute as much as a third of the nitrous oxide in air. However, two chemists now report finding evidence that these studies vastly over-estimate the nitrous oxide coming from combustion of fossil fuels.

According to Lawrence Muzio of the Fossil Energy Research Corp. in Laguna Hills, Calif., and John Kramlich of the Energy and Environmental Research Corp. in Irvine, Calif., a measuring artifact may be creeping into most analyses of furnace exhaust. Because of this, a researcher analyzing exhaust could measure high levels of nitrous oxide even if the gas leaving the furnace contained little or none of it, they report in the November GEOPHYSICAL RESEARCH LETTERS.

"This looks like a major embarrassment in the sense that the research community thought the N.sub.2.O budget was balanced," says Ralph Cicerone, an atmospheric chemist at the National Center for Atmospheric Research in Boulder, Colo.

To analyze the nitrous oxide content of furnace exhaust, researchers have traditionally collected gas inside the furnace, stored the exhaust in a flask, then carried the flask back to the lab for testing. Muzio and Kramlich discovered, though, that while a gas sample sits in the flask, chemical reactions can create nitrous oxide from other components in the exhaust.

While sampling gas from a model furnace, the researchers found that in less than 2 hours, nitrous oxide levels in a stored sample could shoot from less than 5 parts per million to 300 parts per million, if the original exhaust contained nitric oxide (NO), water and sulfur dioxide, all common products of fossil-fuel combustion.

"This fundamentally revises our thinking," says atmospheric chemist Joel Levine from NASA Langley Research Center in Hampton, Va. "If the emission factor in the chimney is reduced by a factor of 100, then coal burning, on a global scheme, does not become a major source of N.sub.2.O."

If fossil-fuel combustion does not account for much of the nitrous oxide in the atmosphere, then some other source must be much more prodigious than previously supposed. Scientists say one possible process making up the difference might be biomass burning--which includes tropical rain forests, grasslands and agricultural stubble. When vegetation burns, the combustion process creates nitrous oxide and other gases.

According to Levine, new work suggests that past studies have underestimated the amount of land burned each year. Moreover, his group and others around the world have recently discovered that burning creates nitrous oxide not only through straight combustion but also by stimulating soil microbes, which produce this gas for months after a fire (SN: 4/9/88, p.231).